The time-honored inspection method of determining distance to an anchorage is fast and simple. For years, seamen have been accustomed to describe arcs on a chart around the anchorage point. A fix, falling near a range arc, shows quickly the distance yet to go.
Determining the course to steer is less simple, and worse, it is quite slow.
Reduced to its elements, the accepted method of determining course and distance to an anchorage requires five steps:
1. Determine the bearings of several landmarks adjacent to the anchorage; from these,
2. Plot the observed bearings on a chart for the area; then
3. Determine the distance to the anchorage by inspection of the position of the fix with respect to previously plotted range arcs; and
4. Position the parallel motion protractor so that its straight edge passes through the fix just plotted and through the drop point of the anchorage.
5. From the vector machine, read off the course to the anchorage.
Upon analysis, it is apparent that distance determination is one step and that course determination is quite another. Both solutions are based on the plotted fix as the common factor, but each requires entirely different methods. To plot, then solve for distance, and finally to determine the course is a prime example of a bottleneck sequence.
A transparent anchoring template can be used which will divide these functions. The fix can be first laid down by a plotter by the familiar method. From the fix, one person as an observer can both read off the distance and predict the course. Meanwhile the plotter, another person, will be able to move on to plot the next bearings without the delay required to read off course and distance.
The accompanying illustration shows the template in position and at work. From it, the method of simultaneously reading distance to the let-go point and the course to steer to the anchorage can be quickly visualized.
The template carries its own drawn-in dead-time diameter.1 This may be readily varied within acceptable limits by inspection as needed since the observer is freed of plotting duties. A range scale for plotting radar information has proven useful in periods of low visibility or where visual bearings are limited.
Parallels are provided at intervals to assist in orienting the template. After the crosshairs are positioned over the anchorage mark on the chart, the horizontals are placed by inspection or by vector machine so that they run east and west with the parallels of the chart. The template can be held in position with thumbtacks or with masking tape.
Bearings are plotted with a grease pencil, called “pencil, china marking” in Supply Corps jargon.
For each chart scale, a new template must be prepared. Once an original drawing to any one scale has been prepared, reproduction for that scale or any other scale is a simple process which can be solved photographically.2
Production cost is small,3 especially if mass production be justified, such as for vessels of similar type or characteristics.
The template has proven satisfactory in use. It substantially cuts prediction time for courses as well as providing the distance, both by observation. It provides faster fixes by permitting the plotter to move on to another fix instead of stopping to solve for course and distance. It permits quick change when a shift of anchorage becomes necessary late in the approach. It saves the time needed to prepare range arcs in advance and increases accuracy by its careful preparation.
1. The dead-time factor includes an adjustment for the distance between the anchor and the navigation bridge from which the bearings are observed. It also can include a factor for dead-time arising from plotting and for a delay between passing the word and the let-go of the anchor. In the illustration, it is set at 150 yards. In practice it is often reduced by observation to as little as 100 yards; if the observer notes that the approach is quite slow, the distance covered during the deadtime will be correspondingly smaller.
2. An original drawing at 1:10,000 serves for any scale by applying a fraction before photographing. Only the indicated scale on the drawing need be changed. The accuracy of the proportions can be proved before the photograph is taken by comparing, with dividers, the image on the ground glass focal plane of the camera with a manually prepared scale covering anticipated fractions, or indeed-by comparison with the scale from a chart itself. The print is made on the reverse side of process film so that pencilling will not damage the photographically reproduced lines.
3. The original drawing required about ten man-hours to prepare. The only remaining time costs are in photographic reproduction, with about one-half hour for adjusting each print to scale and producing the negative. Once each negative has been produced to scale, mass production is feasible. Any good sized ship has photographic equipment available, and smaller vessels can get the templates produced by tender or navy yard at small cost. Type commanders can reproduce these templates at minimal cost, since the only variable is the estimated drop-point circle, probably similar in any given type.